Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/344

 .n'x very clofely conne&ed, which quite furround the mafs of humour, and are immediately applied to the retina, all the way to the great circumference of the corona ciliaris; but from thence to the circular edge of the foffula of the cryftalline, this coat is full of radiated fulci, which contain the proceffus ciliares of the uvea. The internal lamina of the tunica vitrea gives off, through the whole fubftance of this humour, a great number of cellular elongations or fep^a. The radiated fulci of the tunica vitrea, which may be termed fidci ciiiares,, are perfectly black, when the coat is taken out of the,body. The ciyftalline is a fmall lenticular body, of a pretty firm confidence, and tranfparent like cryftal. It is contained in a tranfparent membranous capfula, and lodged in the anterior foflula of the vitreous humour. The figure of the cryftalline is lenticular, but its pofterior fide is more convex than the anterior, the convexity of both fides being very rarely equal. The cryftalline capfula or coat is formed by a duplica ture of the tunica vitrea The external lamina covers the anterior fide of the cryftalline mafs ; the internal lamina covers the backfide, and likewife the foffula vitrea, in which the cryftalline is lodged. The anterior portion fwells when macerated in water, and then appears to be made up of two pellicuke, united by a fine fpungy fubftance. The aqueous humour is a very limpid fluid, refembling a kind of lympha or ferum, with a very fmall degree of vifcidity : and it has no particular capfula like the cryftalline and vitreous humours. It fills the fpace between the cornea lucida and uvea, that between the uvea and the cryftalline, and the hole of the pupilla. Thefe two ipaces are called the chambers of the aqueous humour, and they are diftinguiihed into the anterior and pofterior. The anterior chamber, which is vifible to every body, between the cornea lucida and uvea, is the largeft; the other between the uvea and cryftalline is very narrow, efpecially near the pupilla, where the uvea almoft touches the cryftalline. Tunica Albuginea and Muscles of ths Globe of the Eye. The tunica albuginea, called commonly the •white of the eye, and which appears on all the anterior convex fide of the globe, from the cornea lucida, to the beginning of the pofterior fide, is formed chiefly by the tendinous expanfion of four mufcles. There are commonly fix mufcles inferred in the globe of che human eye, and' they are divided into four re<fti and two obliqui. The redti are again divided, from their fituation, into fuperior, inferior, internal, and external ; and from their fundions, into a levator, depreffor, addudor, and abdudor. The two oblique mufcles are denominated from their fituation and fize, one being named obliquut fuperior or major, the other ohliqttns inferior, or minor. The obliquus major is likewife called trochlearis, becaufe it paffes through a final] cartilaginous ring, as over a trochlea or pulley. The mufculi redi are fixed by their pofterior extremi-

O M Y. Part VL ties at the bottom of the orbit near the foramen opticum in the elongation of the dura mater, by flrort narrow tendons. Fpom thence they run wholly flefhy, toward the great circumference of the convexity of the globe, between the optic nerve and cornea lucida, where they are expanded into flat broad tendons which touch each other, and afterwards unite. Thefe tendons are fixed firft of all by a particular infertion in the circumference juft mentioned, and afterwards continue their adhefion all the way to the cornea, forming the tunica albuginea. The fuperior oblique mufcle is fixed to the bottom of the orbit, by a narrow tendon, in the fame manner as the redi, between the redus fuperior and internus. From thence it runs on the orbit oppofite to the interftice between thefe two mufcles, toward the internal angular apophyfis of the os frontis, where it terminates in a thin tendon, which having paffed through a kind of ring as over a pulley, runs afterwards in a vagina obliquely backward under the redus fuperior, that is between that mufcle and the globe ; and, increafing in breadth, it is inferred pofteriorly and laterally in the globe, near the redus externus. The ring through which this mufcle paffes, is partly cartilaginous and partly ligamentary. The cartilaginous portion is flat, of a confiderable breadth, and like half a ring. The ligamentary portion adheres ftrongly to the two ends of the cartilage, and is fixed in the fmall foffula which lies in the orbit, on the angular apophyfis of the os frontis. By means of this ligament, the ring is in fome meafure moveable, and yields to the motions of the mufcle. To the anterior edge of the ring, a ligamentary vagina is fixed, which invefts the tendon all the way ta its infertion in the globe. The obliquus inferior is fituated obliquely at the lower fide of the orbit, under the redus inferior, which confequently lies between this mufcle and the globe. It is fixed by one extremity a little tendinous, to the root of the nafal apophyfis of the os maxillare, near the edge of the orbit between the opening of the dudus nafaiis, and the inferior orbitary fiffure. From thence it paffes obliquely, and a little tranfvsrfely backward, under the redus inferior, and is fixed in the pofterior lateral part of the globe by a flat tendon, oppofite to, and at a fmall diftance from the tendon of the obliquus fuperior, fo that thefe two mufcles do in fome meafure furround the outer pofterior part of the globe. The redus fuperior moves the anterior portion of the globe upward when we lift up the eyes ; the redus inferior carries this portion downward ; the internus,, toward the nofe; and the externus, toward the temples. When two neighbouring redi ad at the fame time, they carry the anterior portion of the globe obliquely tor ward that fide which anfwers to the diftance between thefe two mufcles ; and when all the four mnfcles ad fiicceflively, they turn the globe of the eye round, which is what is called rolling the eyes. The ufe of the oblique mufcles is chiefly to counterbalance the adion of the redi, and to fupport the globe in all. the motions- already mentioned. This is evident from their